<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
<script src="../../scripts/jquery.js" type="text/javascript"></script>
<script type="text/javascript">
$(function(){
   $("#sub").bind("click",function(event){
         var username = $("#username").val();  //获取元素的值
         if(username==""){     //判断值是否为空
		     $("#msg").html("<p>文本框的值不能为空.</p>");  //提示信息
		     event.preventDefault();  //阻止默认行为 ( 表单提交 )
		 }
   })
})
</script>
</head>
<body>
<form action="test.html">
用户名：<input type="text" id="username" />
<br/>
<input type="submit" value="提交" id="sub"/>
</form>

<div id="msg"></div>
</body>
</html>